The present invention is applied in the context of petroleum and petrochemical processes wherein substantial energy is consumed in the pumping of liquids. Examples of such applications are the pumping of liquid feeds to elevated pressures and recycle processes in which relatively large volumes of liquids are circulated within the process. The invention is particularly relevant wherein the pumping rate is variable, with resulting inefficiency in the energy requirement.
One specific application is in continuous-separation processes for the selective adsorption of an extract from a mixture comprising a raffinate and/or another byproduct. Such processes are in widespread use for the separation of hydrocarbons, for example the separation of para-xylene and/or meta-xylene from a mixture of C8 aromatics, normal paraffins from a paraffin mixture, or specific olefins from a mixture of olefins and paraffins. Generally, the processes use a solid adsorbent which preferentially retains the extract in order to separate the extract from the rest of the mixture.
The solid adsorbent often is in the form of a simulated moving bed, wherein the bed of solid adsorbent is held stationary and the locations at which the various streams enter and leave the bed are periodically moved. The adsorbent bed itself is usually a succession of fixed sub-beds. The shift in the locations of liquid input and output in the direction of the fluid flow through the bed simulates the movement of the solid adsorbent in the opposite direction. The shift in locations of liquid input and output is accomplished by a fluid directing device known generally as a rotary valve which works in conjunction with distributors located between the adsorbent sub-beds. A circulating stream is conducted by pumps circulating liquid from the bottom to the top of the bed of adsorbent. The composition of the circulating stream changes with each valve step. The bed of adsorbent may be contained in two or more chambers, with corresponding numbers of circulating streams and pumps. The circulating pumps, moving substantial and varying quantities of material around one or multiple adsorbent chambers, are significant energy consumers.
For greater detail regarding the simulated moving bed and its operation, see U.S. Pat. No. 2,985,589. Relevant to a para-xylene separation process; see Mowry, J. R. In Handbook of Petroleum Refining Processes; Meyers, R. A. Ed.; McGraw-Hill: New York, 1986; pp 8-79 to 8-99.